This application is based on and claims priority under 35 U.S.C. xc2xa7119 with respect to Japanese Application No. 2001-094394 filed on Mar. 28, 2001, the entire content of which is incorporated herein by reference.
The present invention generally relates to a pipe mechanism. More particularly, the present invention pertains to a pipe mechanism for an exhaust gas recirculation system which returns a part of the exhaust gas of an internal combustion engine from an exhaust pipe into the intake pipe.
An example of a known pipe mechanism for an exhaust gas recirculation system is disclosed in Japanese Patent No. 2582966. This known pipe mechanism has an inner tube and an outer tube, both of which have an upstream end and a downstream end. Both of the downstream end portions are connected to each other, and the downstream end portions are inserted into the intake pipe of the internal combustion engine. The upstream end portion of the inner tube is connected with an exhaust pipe so that a part of the exhaust gas flows from the exhaust pipe into the inner tube. A predetermined clearance exists between the outer circumference of the inner tube and the inner circumference of the outer tube. The upstream end portion of the outer tube forms an opening to introduce fresh air into the predetermined clearance. Thus, the predetermined clearance acts as a heat insulating wall to cool the exhaust gas in the inner tube.
However, the upstream end portion of the inner tube is connected with the exhaust pipe, and the outer circumference of the outer tube is connected with the intake pipe. These connecting portions and the connection between the inner tube and the outer tube receive concentrated stress in the known pipe mechanism. Thus, in the known pipe mechanism, the connecting portions and the connection have to be sufficiently strong to withstand the stress.
If the thickness of both the inner tube and the outer tube is increased to increase the strength for purposes of withstanding the stress, the heat conduction of the inner and outer tubes is improved. The heat of the exhaust gas in the inner tube will be transferred to the outer tube. However, this is not desirable because the outer tube is attached to the intake pipe which is made of resin. If the outer tube is heated, the heat resistance of the intake pipe has to be improved. Therefore, increasing the thickness of both the inner tube and the outer tube can create other problems.
In addition, because the inner tube is fixed to the exhaust pipe in the known pipe mechanism, the heat in the exhaust pipe is transferred to the intake pipe via the inner tube. Thus, another problem exists in that the intake air may be excessively heated.
According to one aspect, a pipe mechanism for an exhaust gas recirculation system includes a joint pipe connected to an exhaust pipe, an inner tube through which flows exhaust gas and an outer tube. The outer tube has an upstream end and a downstream end, with the downstream end being positioned in an intake pipe. The outer tube has an upstream end connected with the joint pipe and a downstream end attached to the outer surface of the inner tube. The outer tube is located around the inner tube with a predetermined clearance.
According to another aspect, a pipe mechanism in an exhaust gas recirculation system includes a joint pipe connected to an exhaust pipe, a main pipe positioned between the joint pipe and an intake pipe, with the main pipe being comprised of an inner tube through which exhaust gas is adapted to flow and an outer tube. The inner surface of the outer tube faces the outer surface of the inner tube, and the inner surface of the outer rube is spaced from the outer surface of the inner tube over at least a portion of the length of the main pipe to provide a clearance. The inner and outer tubes each having an upstream end portion and a downstream end portion, with the downstream end portion of the inner tube being connected to the downstream end portion of the outer tube at a connection portion. The upstream end portion of the outer tube is connected to the joint pipe.